Twice today I’ve seen problems with ArcGIS 10 (and 10.1) failing to launch. In one instance, no specific application error was given – ArcMap just failed to start with a ‘serious error’ dialog, and in the other, some direction was provided by ArcMap in the form of an error message.

In both cases, the user had a corrupt default user geodatabase. The location of the default geodatabase may be given in the error message. If not, it should be located in <user profile>\My Documents\ArcGIS. If you are working on a lab machine, the location will be h:\ArcGIS\  The file is named Default.gdb, and if ArcMap has had trouble loading more than once, you may have multiple copies named Default.gdb, Default.gdb1, Default.gdb2…

The ESRI knowledge base article 38523 suggests several fixes, the most important of which seems to be deleting any Default.gdb files in the users profile directory. I found that using Windows worked properly, as ArcCatalog seems unable to remove the offending files. One machine I worked on today had more than 150 default.gdb files (all of which were corrupt and preventing ArcMap from launching).